There is a growing need for a flat-panel display as an image display device with the wide spreading use of information terminals. In addition, there are increasing opportunities, in which information, which has been conventionally provided by paper medium, is digitized with further advancement of informatization. Particularly, the needs for an electronic paper or a digital paper as a thin and light weight mobile display media, which can be easily held and carried, have recently increased (see patent document 1).
Generally, in a flat panel display device, its display medium is formed by using an element such as a liquid crystal, an organic EL (organic electroluminescence), and an electrophoresis. In such display media, a technology which uses an active drive element (TFT element) as an image drive element has become mainstream in order to secure the homogeneity of screen luminosity and screen rewriting speed and so forth. In the conventional display device, such TFT elements are formed on a glass substrate, and thereafter a liquid crystal element or an organic EL element and so forth is sealed.
As a TFT element, semiconductors including a-Si (amorphous silicon) and p-Si (polysilicon) can be mainly used. These Si semiconductors (together with metal films, as necessary) are multilayered, and each of a source electrode, a drain electrode and a gate electrode is sequentially stacked on a substrate, to thereby manufacture a TFT element. In such manufacturing of a TFT element, a manufacturing process using a vacuum system such as sputtering and so forth is usually required.
That is, in manufacturing a TFT element using those existing manufacturing facilities, it has been necessitated to form individual layers by applying a manufacturing process using a vacuum system including a vacuum chamber over and over again, so that the costs of facilities and operation have become quite expensive. For example, in manufacturing a TFT element, it is necessary to apply each process of vacuum film formation including vacuum deposition, doping, photolithography or developing over and over again to form each layer, so that an element is formed on a substrate through several tens of processes. In these manufacturing processes using conventional Si semiconductors, it is not easy to change facility designs when an extensive design change of the manufacturing device such as a vacuum chamber is required in response to the need to increase the size of a display screen.
In addition, the conventional process of manufacturing a TFT element using Si materials includes one or more steps using a high temperature, so that an additional restriction that the material of the substrate should resist a high process temperature is added. For this reason, there is no way to select a glass as the material of the substrate in practice. Thus, when a thin display device such as an electronic paper or a digital paper is constructed by using the TFT elements which were conventionally known, such a display device comes to have a heavy weight and lacks flexibility. Additionally, there is a possibility that such a display device will break due to a shock when it falls, since it is made of a glass substrate. These problems, which arose from the formation of a TFT element on a glass substrate, are not desirable when it is necessary to meet the need for a portable display that is thin and light weight with the advancement of informatization.
On the other hand, research on organic semiconductor materials as organic compounds which have high charge transporting properties have been energetically developed in recent years. These compounds are expected to be applied to an organic laser oscillating element and an organic thin film transistor element (an organic TFT element) besides an electric charge transporting material for an organic EL device.
When a device using these organic semiconductors is realized, simplification of the manufacturing process using vacuum or low pressure deposition at a relatively low temperature will be attained.
It has been considered impossible to apply a manufacturing process using low temperature to the conventional Si based semiconducting material. However, it seems to be possible that a device using an organic semiconductor can be manufactured by a manufacturing process using low temperature. Therefore, the restriction regarding the above-mentioned heat resistance of the substrate would be relieved, so that a TFT element may be formed on, for example, a transparent resin substrate. Further, if a TFT element may be formed on a transparent resin substrate and a display material may be driven by the TFT element, it is possible to manufacture a display device which has a lighter weight than the conventional display devices. In addition, it is also possible to provide a display device which has a superior flexibility and which does not break (or hardly breaks) when it falls.